The present invention relates to methods of making articles which are constituted by delicate structures contained within sealed housings, and to products so constituted. More particularly, the invention pertains to methods of making integrated circuit chip assemblies ("packages") each constituted by an integrated chip within a sealed housing formed partly of injection molded plastic, with leads extending in sealed relation through the housing for electrical connections, and to the products so constituted. Since the invention is likely to find its most advantageous usage in the manufacture of integrated circuit chip assemblies, the latter will be referred to in the following disclosure as a specific example of the generic applications of the invention. For generic breadth, limited reference will be made to "delicate structures" of the sort typified by integrated circuit chips and fine connecting wires.
As the solid state electronics industry has progressed during the last several years, the numerical volume of integrated circuit chip assemblies manufactured and sold each month has risen astronomically. The "scale of integration" (in terms of the number of equivalent circuit elements on one chip) has increased in like fashion. There are literally millions of integrated chip assemblies now being manufactured each month, and while some may be quite simple (e.g., "quad flipflops"), others are very extensive and/or expensive (e.g., 32-bit microprocessors or one megabit memories) in terms of the complexity and equivalent number of individual circuit elements on a single chip. As the chips per se are being manufactured and tested, there are necessarily some which are found to be defective and thus rejected; "yield" is significantly less than one hundred percent. The second major stage of manufacture involves "packaging" each chip in a sealed housing with electrical leads extending therefrom.
It will be useful to define and distinguish between two broad classes of integrated chip packages, i.e., those commonly called "commercial grade" and "MIL SPEC", respectively. The "commercial grade" assemblies most usually include hermetically sealed housings constituted in whole or in part by molded plastic materials, whereas MIL SPEC assemblies most usually include hermetically sealed housings constituted by ceramic material walls joined by fused glass frit seals fired (melted) at very high temperatures. The MIL SPEC assemblies have much higher immunization to environmental extremes of moisture, temperatures, pressure and vibration which may be encountered in military or outer space applications. The "commercial grade" products are neither constructed nor rated to stand up under such extremes, but must be and are capable of preserving the interior chip under the less severe environmental conditions encountered in industrial, commercial, and domestic consumer uses. MIL SPEC grade chip assemblies are much more expensive--in terms of cost for both the housing materials and the manufacturing procedures--, and they are manufactured in much lesser quantities compared to the "commercial grade". For purposes of the present patent application, however, the two classes of products will be called Type I (exemplified by the "commercial grade") to designate that at least some portion of the sealed housing is constituted by injection molded plastic material, and Type II (exemplified by "MIL SPEC grade") to designate that no portion of the sealed housing is formed of injection molded plastic. These definitions are to be taken without regard to ratings or capabilities for survival in military or outer space applications (on the one hand) or only industrial/commercial applications (on the other hand).
Type I integrated circuit assemblies or packages are truly "mass produced" in view of the market demand and the quantities turned out each month. The process of "packaging" chip assemblies of Type I must be carried out on every product and thus is practiced literally millions of times. This second stage operation also results (for reasons explained below) in some of such final products being defective and thus rejected; "yield" from the second stage of Type I assembly manufacturing is significantly less than one hundred percent and is estimated to be on the order of ninety to ninety-five percent.
A defective packaged assembly almost always means discarding a previously tested "good" integrated chip at the most costly stage. Thus, rejects arising during and because of the packaging operation constitute a severe burden on the cost and efficiency of manufacturing integrated circuit assemblies. The problem is all the more severe when packaging operations result in scrapping of relatively expensive and "large scale" integrated chips (e.g., 32-bit microprocessors).